Reestablishing synchronization in a memory system

ABSTRACT

Embodiments relate to reestablishing synchronization across multiple channels in a memory system. One aspect is a system that includes a plurality of channels, each providing communication with a memory buffer chip and a plurality of memory devices. A memory control unit is coupled to the plurality of channels. The memory control unit is configured to perform a method that includes receiving an out-of-synchronization indication associated with at least one of the channels. The memory control unit performs a first stage of reestablishing synchronization that includes selectively stopping new traffic on the plurality of channels, waiting for a first time period to expire, resuming traffic on the plurality of channels based on the first time period expiring, and verifying that synchronization is reestablished for a second time period.

BACKGROUND

The present invention relates generally to computer memory, and morespecifically, to reestablishing synchronization across multiple channelsin a memory system.

Contemporary high performance computing main memory systems aregenerally composed of one or more memory devices, which are connected toone or more memory controllers and/or processors via one or more memoryinterface elements such as buffers, hubs, bus-to-bus converters, etc.The memory devices are generally located on a memory subsystem such as amemory card or memory module and are often connected via a pluggableinterconnection system (e.g., one or more connectors) to a system board(e.g., a PC motherboard).

Overall computer system performance is affected by each of the keyelements of the computer structure, including the performance/structureof the processor(s), any memory cache(s), the input/output (I/O)subsystem(s), the efficiency of the memory control function(s), theperformance of the main memory devices(s) and any associated memoryinterface elements, and the type and structure of the memoryinterconnect interface(s).

Extensive research and development efforts are invested by the industry,on an ongoing basis, to create improved and/or innovative solutions tomaximizing overall system performance and density by improving thememory system/subsystem design and/or structure. High-availabilitysystems present further challenges as related to overall systemreliability due to customer expectations that new computer systems willmarkedly surpass existing systems in regard to mean-time-between-failure(MTBF), in addition to offering additional functions, increasedperformance, increased storage, lower operating costs, etc. Otherfrequent customer requirements further exacerbate the memory systemdesign challenges, and include such items as ease of upgrade and reducedsystem environmental impact (such as space, power and cooling). Inaddition, customers are requiring the ability to access an increasingnumber of higher density memory devices (e.g., DDR3 and DDR4 SDRAMs) atfaster and faster access speeds.

In a high-availability memory subsystem, a memory controller typicallycontrols multiple memory channels, where each memory channel has one ormore dual in-line memory modules (DIMMs) that include dynamic randomaccess memory (DRAM) devices and in some instances a memory buffer chip.The memory buffer chip typically acts as a slave device to the memorycontroller, reacting to commands provided by the memory controller. Thememory subsystem can be configured as a redundant array of independentmemory (RAIM) system to support recovery from failures of either DRAMdevices or an entire channel. In RAIM, data blocks are striped acrossthe channels along with check bit symbols and redundancy information.Examples of RAIM systems may be found, for instance, in U.S. PatentPublication Number 2011/0320918 titled “RAIM System Using Decoding ofVirtual ECC”, filed on Jun. 24, 2010, the contents of which are herebyincorporated by reference in its entirety, and in U.S. PatentPublication Number 2011/0320914 titled “Error Correction and Detectionin a Redundant Memory System”, filed on Jun. 24, 2010, the contents ofwhich are hereby incorporated by reference in its entirety.

In a RAIM system, data is typically returned from all memory channels ofthe memory subsystem at close to the same time. When one channel issignificantly later than the others, overall memory latency is increasedby having to wait for that channel's data to return prior to sending adata block from the channels to a cache subsystem. The memory controllertypically issues commands in lock-step synchronization across allchannels to maintain synchronization. In the presence of an error on achannel, the memory controller corrects fetched data and initiates arecovery sequence. A recovering channel is thrown out of synchronizationwith the other channels. The system may be forced to wait for therecovering channel's data for subsequent data block transfers where therecovery sequence itself is not managed by the memory controller.

SUMMARY

Embodiments include a method, system, and computer program product forreestablishing synchronization across multiple channels in a memorysystem. A system for reestablishing synchronization across multiplechannels in a memory system includes a plurality of channels, eachproviding communication with a memory buffer chip and a plurality ofmemory devices. A memory control unit is coupled to the plurality ofchannels. The memory control unit is configured to perform a method thatincludes receiving an out-of-synchronization indication associated withat least one of the channels. The memory control unit performs a firststage of reestablishing synchronization that includes selectivelystopping new traffic on the plurality of channels, waiting for a firsttime period to expire, resuming traffic on the plurality of channelsbased on the first time period expiring, and verifying thatsynchronization is reestablished for a second time period.

A computer implemented method for reestablishing synchronization acrossmultiple channels in a memory system includes receiving anout-of-synchronization indication associated with at least one of aplurality of channels in the memory system. A memory control unit incommunication with the channels performs a first stage of reestablishingsynchronization that includes selectively stopping new traffic on theplurality of channels, waiting for a first time period to expire,resuming traffic on the plurality of channels based on the first timeperiod expiring, and verifying that synchronization is reestablished fora second time period.

A computer program product for reestablishing synchronization acrossmultiple channels in a memory system is provided. The computer programproduct includes a tangible storage medium readable by a processingcircuit and storing instructions for execution by the processing circuitfor performing a method. The method includes receiving anout-of-synchronization indication associated with at least one of aplurality of channels in the memory system. A memory control unit incommunication with the channels performs a first stage of reestablishingsynchronization that includes selectively stopping new traffic on theplurality of channels, waiting for a first time period to expire,resuming traffic on the plurality of channels based on the first timeperiod expiring, and verifying that synchronization is reestablished fora second time period.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The subject matter which is regarded as embodiments is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe embodiments are apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 depicts a memory system in accordance with an embodiment;

FIG. 2 depicts a memory subsystem in a planar configuration inaccordance with an embodiment;

FIG. 3 depicts a memory subsystem in a buffered DIMM configuration inaccordance with an embodiment;

FIG. 4 depicts a memory subsystem with dual asynchronous and synchronousmemory operation modes in accordance with an embodiment;

FIG. 5 depicts a memory subsystem channel and interfaces in accordancewith an embodiment;

FIG. 6 depicts a process flow for providing synchronous operation in amemory subsystem in accordance with an embodiment;

FIG. 7 depicts a process flow for establishing alignment between nestand memory domains in a memory subsystem in accordance with anembodiment;

FIG. 8 depicts a timing diagram of synchronizing a memory subsystem inaccordance with an embodiment;

FIG. 9A depicts a memory control unit in accordance with an embodiment;

FIG. 9B depicts a data tag table and a done tag table for a memorycontrol unit in accordance with an embodiment;

FIGS. 10A, 10B, and 10C depict a process for reestablishingsynchronization across multiple channels in a memory system inaccordance with an embodiment; and

FIG. 11 illustrates a computer program product in accordance with anembodiment.

DETAILED DESCRIPTION

Exemplary embodiments provide a system, method, and computer programproduct for reestablishing synchronization across multiple channels in amemory system. The memory system includes a processing subsystem thatcommunicates synchronously with a memory subsystem in a nest domain. Thememory subsystem also includes a memory domain that can be runsynchronously or asynchronously relative to the nest domain. The memorysystem includes a memory controller that interfaces with the memorysubsystem having multiple memory channels. The memory controllerincludes an out-of-sync/out-of-order detector to detectout-of-synchronization and out-of-order conditions between the memorychannels while tolerating a degree of skew within the memory system.Upon detecting misalignment beyond a threshold amount, actions can betaken to re-align and synchronize the memory channels. The memorycontroller attempts to reestablish synchronization without havingcontrol of the underlying replay system used for recovery.

In an exemplary embodiment, a programmable quiesce sequence is used toincrementally attempt to restore channel synchronization by stoppingstores and other downstream commands over a programmable time interval.The sequence may wait for completion indications from a memory bufferchip of the recovering channel and additionally inject synchronizationcommands across all channels. The state of synchronization may bechecked before exiting the sequence, and if a channel remains out ofsynchronization, the quiesce sequence can be retried under programmaticcontrol. The memory controller can use the quiesce sequence withoutmodifications to underlying channel recovery features.

FIG. 1 depicts an example memory system 100 which may be part of alarger computer system structure. A control processor (CP) system 102 isa processing subsystem that includes at least one processor 104configured to interface with a memory control unit (MCU) 106. Theprocessor 104 can be a multi-core processor or module that processesread, write, and configuration requests from a system controller (notdepicted). The MCU 106 includes a memory controller synchronous (MCS)108, also referred to as a memory controller, that controlscommunication with a number of channels 110 for accessing a plurality ofmemory devices in a memory subsystem 112. The MCU 106 and the MCS 108may include one or more processing circuits, or processing may beperformed by or in conjunction with the processor 104. In the example ofFIG. 1, there are five channels 110 that can support parallel memoryaccesses as a virtual channel 111. In an embodiment, the memory system100 is a five-channel redundant array of independent memory (RAIM)system, where four of the channels 110 provide access to columns of dataand check-bit memory, and a fifth channel provides access to RAIM paritybits in the memory subsystem 112.

Each of the channels 110 is a synchronous channel which includes adownstream bus 114 and an upstream bus 116. Each downstream bus 114 of agiven channel 110 may include a different number of lanes or links thana corresponding upstream bus 116. In the example of FIG. 1, eachdownstream bus 114 includes n-unidirectional high-speed serial lanes andeach upstream bus 116 includes m-unidirectional high-speed serial lanes.Frames of commands and/or data can be transmitted and received on eachof the channels 110 as packets that are decomposed into individual lanesfor serial communication. In an embodiment, packets are transmitted atabout 9.6 gigabits per second (Gbps), and each transmitting lanetransmits four-bit groups serially per channel 110. The memory subsystem112 receives, de-skews, and de-serializes each four-bit group per laneof the downstream bus 114 to reconstruct a frame per channel 110 fromthe MCU 106. Likewise, the memory subsystem 112 can transmit to the MCU106 a frame of packets as four-bit groups per lane of the upstream bus116 per channel 110. Each frame can include one or more packets, alsoreferred to as transmission packets.

The CP system 102 may also include a cache subsystem 118 that interfaceswith the processor 104. A cache subsystem interface 122 of the CP system102 provides a communication interface to the cache subsystem 118. Thecache subsystem interface 122 may receive data from the memory subsystem112 via the MCU 106 to store in the cache subsystem 118.

FIG. 2 depicts an example of a memory subsystem 112 a as an instance ofthe memory subsystem 112 of FIG. 1 in a planar configuration 200 inaccordance with an embodiment. The example of FIG. 2 only depicts onechannel 110 of the memory subsystem 112 a; however, it will beunderstood that the memory subsystem 112 a can include multipleinstances of the planar configuration 200 as depicted in FIG. 2, e.g.,five instances. As illustrated in FIG. 2, the planar configuration 200includes a memory buffer chip 202 connected to a plurality of dynamicrandom access memory (DRAM) devices 204 via connectors 206. The DRAMdevices 204 may be organized as ranks of one or more dual in-line memorymodules (DIMMs) 208. The each of the connectors 206 is coupled to adouble data rate (DDR) port 210, also referred to as a memory interfaceport 210 of the memory buffer chip 202, where each DDR port 210 can becoupled to more than one connector 206. In the example of FIG. 2, thememory buffer chip 202 includes DDR ports 210 a, 210 b, 210 c, and 210d. The DDR ports 210 a and 210 b are each coupled to a pair ofconnectors 206 and a shared memory buffer adaptor (MBA) 212 a. The DDRports 210 c and 210 d may each be coupled to a single connector 206 anda shared memory buffer adaptor (MBA) 212 b. The DDR ports 210 a-210 dare JEDEC-compliant memory interfaces for issuing memory commands andreading and writing memory data to the DRAM devices 204.

The MBAs 212 a and 212 b include memory control logic for managingaccesses to the DRAM devices 204, as well as controlling timing,refresh, calibration, and the like. The MBAs 212 a and 212 b can beoperated in parallel, such that an operation on DDR port 210 a or 210 bcan be performed in parallel with an operation on DDR port 210 c or 210d.

The memory buffer chip 202 also includes an interface 214 configured tocommunicate with a corresponding interface 216 of the MCU 106 via thechannel 110. Synchronous communication is established between theinterfaces 214 and 216. As such, a portion of the memory buffer chip 202including a memory buffer unit (MBU) 218 operates in a nest domain 220which is synchronous with the MCS 108 of the CP system 102. A boundarylayer 222 divides the nest domain 220 from a memory domain 224. The MBAs212 a and 212 b and the DDR ports 210 a-210 d, as well as the DRAMdevices 204 are in the memory domain 224. A timing relationship betweenthe nest domain 220 and the memory domain 224 is configurable, such thatthe memory domain 224 can operate asynchronously relative to the nestdomain 220, or the memory domain 224 can operate synchronously relativeto the nest domain 220. The boundary layer 222 is configurable tooperate in a synchronous transfer mode and an asynchronous transfer modebetween the nest and memory domains 220, 224. The memory buffer chip 202may also include one or more multiple-input shift-registers (MISRs) 226,as further described herein. For example, the MBA 212 a can include oneor more MISR 226 a, and the MBA 212 b can include one or more MISR 226b. Other instances of MISRs 226 can be included elsewhere within thememory system 100. As a further example, one or more MISRs 226 can bepositioned individually or in a hierarchy that spans the MBU 218 andMBAs 212 a and 212 b and/or in the MCU 106.

The boundary layer 222 is an asynchronous interface that permitsdifferent DIMMs 208 or DRAM devices 204 of varying frequencies to beinstalled into the memory domain 224 without the need to alter thefrequency of the nest domain 220. This allows the CP system 102 toremain intact during memory installs or upgrades, thereby permittinggreater flexibility in custom configurations. In the asynchronoustransfer mode, a handshake protocol can be used to pass commands anddata across the boundary layer 222 between the nest and memory domains220, 224. In the synchronous transfer mode, timing of the memory domain224 is phase adjusted to align with the nest domain 220 such that aperiodic alignment of the nest and memory domains 220, 224 occurs at analignment cycle in which commands and data can cross the boundary layer222.

The nest domain 220 is mainly responsible for reconstructing anddecoding the source synchronous channel packets, applying any necessaryaddressing translations, performing coherency actions, such as directorylook-ups and cache accesses, and dispatching memory operations to thememory domain 224. The memory domain 224 may include queues, ascheduler, dynamic power management controls, hardware engines forcalibrating the DDR ports 210 a-210 d, and maintenance, diagnostic, andtest engines for discovery and management of correctable anduncorrectable errors. There may be other functions in the nest or memorydomain. For instance, there may be a cache of embedded DRAM (eDRAM)memory with a corresponding directory. If the cache is created for someapplications and other instances do not use it, there may be powersavings by connecting a special array voltage (e.g., VCS) to ground.These functions may be incorporated within the MBU 218 or locatedelsewhere within the nest domain 220. The MBAs 212 a and 212 b withinthe memory domain 224 may also include logic to initiate autonomicmemory operations for the DRAM devices 204, such as refresh and periodiccalibration sequences in order to maintain proper data and signalintegrity.

FIG. 3 depicts a memory subsystem 112 b as an instance of the memorysubsystem 112 of FIG. 1 in a buffered DIMM configuration 300 inaccordance with an embodiment. The buffered DIMM configuration 300 caninclude multiple buffered DIMMs 302 within the memory subsystem 112 b,e.g., five or more instances of the buffered DIMM 302, where a singlebuffered DIMM 302 is depicted in FIG. 3 for purposes of explanation. Thebuffered DIMM 302 includes the memory buffer chip 202 of FIG. 2. As inthe example of FIG. 2, the MCS 108 of the MCU 106 in the CP system 102communicates synchronously on channel 110 via the interface 216. In theexample of FIG. 3, the channel 110 interfaces to a connector 304, e.g.,a socket, that is coupled to a connector 306 of the buffered DIMM 302. Asignal path 308 between the connector 306 and the interface 214 of thememory buffer chip 202 enables synchronous communication between theinterfaces 214 and 216.

As in the example of FIG. 2, the memory buffer chip 202 as depicted inFIG. 3 includes the nest domain 220 and the memory domain 224. Similarto FIG. 2, the memory buffer chip 202 may include one or more MISRs 226,such as one or more MISR 226 a in MBA 212 a and one or more MISR 226 bin MBA 212 b. In the example of FIG. 3, the MBU 218 passes commandsacross the boundary layer 222 from the nest domain 220 to the MBA 212 aand/or to the MBA 212 b in the memory domain 224. The MBA 212 ainterfaces with DDR ports 210 a and 210 b, and the MBA 212 b interfaceswith DDR ports 210 c and 210 d. Rather than interfacing with DRAMdevices 204 on one or more DIMMs 208 as in the planar configuration 200of FIG. 2, the DDR ports 210 a-210 d can interface directly with theDRAM devices 204 on the buffered DIMM 302.

The memory subsystem 112 b may also include power management logic 310that provides a voltage source for a voltage rail 312. The voltage rail312 is a local cache voltage rail to power a memory buffer cache 314.The memory buffer cache 314 may be part of the MBU 218. A power selector316 can be used to determine whether the voltage rail 312 is sourced bythe power management logic 310 or tied to ground 318. The voltage rail312 may be tied to ground 318 when the memory buffer cache 314 is notused, thereby reducing power consumption. When the memory buffer cache314 is used, the power selector 316 ties the voltage rail 312 to avoltage supply of the power management logic 310. Fencing and clockgating can also be used to better isolate voltage and clock domains.

As can be seen in reference to FIGS. 2 and 3, a number of memorysubsystem configurations can be supported in embodiments. Varying sizesand configurations of the DRAM devices 204 can have different addressformat requirements, as the number of ranks and the overall details ofslots, rows, columns, banks, bank groups, and/or ports may vary acrossdifferent DRAM devices 204 in embodiments. Various stackingarchitectures (for example, 3 die stacking, or 3DS) may also beimplemented, which may include master ranks and slave ranks in thepackaging architecture. Each of these different configurations of DRAMdevices 204 may require a unique address mapping table. Therefore,generic bits may be used by the MCU 106 to reference particular bits ina DRAM device 204 without having full knowledge of the actual DRAMtopology, thereby separating the physical implementation of the DRAMdevices 204 from the MCU 106. The memory buffer chip 202 may map thegeneric bits to actual locations in the particular type(s) of DRAM thatis attached to the memory buffer chip 202. The generic bits may beprogrammed to hold any appropriate address field, including but notlimited to memory base address, rank (including master or slave), row,column, bank, bank group, and/or port, depending on the particularcomputer system.

FIG. 4 depicts a memory subsystem 112 c as an instance of the memorysubsystem 112 of FIG. 1 with dual asynchronous and synchronous memoryoperation modes in accordance with an embodiment. The memory subsystem112 c can be implemented in the planar configuration 200 of FIG. 2 or inthe buffered DIMM configuration 300 of FIG. 3. As in the examples ofFIGS. 2 and 3, the MCS 108 of the MCU 106 in the CP system 102communicates synchronously on channel 110 via the interface 216. FIG. 4depicts multiple instances of the interface 216 as interfaces 216 a-216n which are configured to communicate with multiple instances of thememory buffer chip 202 a-202 n. In an embodiment, there are five memorybuffer chips 202 a-202 n per CP system 102.

As in the examples of FIGS. 2 and 3, the memory buffer chip 202 a asdepicted in FIG. 4 includes the nest domain 220 and the memory domain224. Also similar to FIGS. 2 and 3, the memory buffer chip 202 a mayinclude one or more MISRs 226, such as one or more MISR 226 a in MBA 212a and one or more MISR 226 b in MBA 212 b. In the example of FIG. 4, theMBU 218 passes commands across the boundary layer 222 from the nestdomain 220 to the MBA 212 a and/or to the MBA 212 b in the memory domain224. The MBA 212 a interfaces with DDR ports 210 a and 210 b, and theMBA 212 b interfaces with DDR ports 210 c and 210 d. The nest domain 220and the memory domain 224 are established and maintained usingphase-locked loops (PLLs) 402, 404, and 406.

The PLL 402 is a memory controller PLL configured to provide a masterclock 408 to the MCS 108 and the interfaces 216 a-216 n in the MCU 106of the CP system 102. The PLL 404 is a nest domain PLL that is coupledto the MBU 218 and the interface 214 of the memory buffer chip 202 a toprovide a plurality of nest domain clocks 405. The PLL 406 is a memorydomain PLL coupled the MBAs 212 a and 212 b and to the DDR ports 210a-210 d to provide a plurality of memory domain clocks 407. The PLL 402is driven by a reference clock 410 to establish the master clock 408.The PLL 404 has a reference clock 408 for synchronizing to the masterclock 405 in the nest domain 220. The PLL 406 can use a separatereference clock 414 or an output 416 of the PLL 404 to provide areference clock 418. The separate reference clock 414 operatesindependent of the PLL 404.

A mode selector 420 determines the source of the reference clock 418based on an operating mode 422 to enable the memory domain 224 to runeither asynchronous or synchronous relative to the nest domain 220. Whenthe operating mode 422 is an asynchronous operating mode, the referenceclock 418 is based on the reference clock 414 as a reference clocksource such that the PLL 406 is driven by separate reference clock and414. When the operating mode 422 is a synchronous operating mode, thereference clock 418 is based on the output 416 of an FSYNC block 492which employs PLL 404 as a reference clock source for synchronous clockalignment. This ensures that the PLLs 404 and 406 have related clocksources based on the reference clock 408. Even though the PLLs 404 and406 can be synchronized in the synchronous operating mode, the PLLs 404and 406 may be configured to operate at different frequencies relativeto each other. Additional frequency multiples and derivatives, such asdouble rate, half rate, quarter rate, etc., can be generated based oneach of the multiplier and divider settings in each of the PLLs 402,404, and 406. For example, the nest domain clocks 405 can includemultiples of a first frequency of the PLL 404, while the memory domainclocks 407 can include multiples of a second frequency of the PLL 406.

In an asynchronous mode of operation each memory buffer chip 202 a-202 nis assigned to an independent channel 110. All data for an individualcache line may be self-contained within the DRAM devices 204 of FIGS. 2and 3 attached to a common memory buffer chip 202. This type ofstructure lends itself to lower-end cost effective systems which canscale the number of channels 110 as well as the DRAM speed and capacityas needs require. Additionally, this structure may be suitable inhigher-end systems that employ features such as mirroring memory on dualchannels 110 to provide high availability in the event of a channeloutage.

When implemented as a RAIM system, the memory buffer chips 202 a-202 ncan be configured in the synchronous mode of operation. In a RAIMconfiguration, memory data is striped across multiple physical memorychannels 110, e.g., five channels 110, which can act as the singlevirtual channel 111 of FIG. 1 in order to provide error-correcting code(ECC) protection for continuous operation, even when an entire channel110 fails. In a RAIM configuration, all of the memory buffer chips 202a-202 n of the same virtual channel 111 are operated synchronously sinceeach memory buffer chip 202 is responsible for a portion of a coherentline.

To support and maintain synchronous operation, the MCU 106 can detectsituations where one channel 110 becomes temporarily or permanentlyincapacitated, thereby resulting in a situation wherein the channel 110is operating out of sync with respect to the other channels 110. In manycases the underlying situation is recoverable, such as intermittenttransmission errors on one of the interfaces 216 a-216 n and/orinterface 214 of one of more of the memory buffer chips 202 a-202 n.Communication on the channels 110 may utilize a robust cyclic redundancycode (CRC) on transmissions, where a detected CRC error triggers arecovery retransmission sequence. There are cases where theretransmission requires some intervention or delay between the detectionand retransmission. A replay system including replay buffers for each ofthe channels can be used to support a recovery retransmission sequencefor a faulty channel 110. Portions of the replay system may be suspendedfor a programmable period of time to ensure that source data to bestored in the replay buffer has been stored prior to initiatingautomated recovery. The period of time while replay is suspended canalso be used to make adjustments to other subsystems, such as voltagecontrols, clocks, tuning logic, power controls, and the like, which mayassist in preventing a recurrence of an error condition that led to thefault. Suspending replay may also remove the need for the MCU 106 toreissue a remaining portion of a store on the failing channel 110 andmay increase the potential of success upon the replay.

Although the recovery retransmission sequence can eventually restore afaulty channel 110 to fully operational status, the overall memorysubsystem 112 remains available during a recovery period. Tolerating atemporary out of sync condition allows memory operations to continue byusing the remaining good (i.e., non-faulty) channels 110 until therecovery sequence is complete. For instance, if data has already startedto transfer back to the cache subsystem 118 of FIG. 1, there may need tobe a way to process failing data after it has been transmitted. Whilereturning data with gaps is one option, another option is to delay thestart of data transmission until all error status is known. Delaying maylead to reduced performance when there is a gapless requirement. Afterrecovering a faulty channel 110, the MCU 106 resynchronizes therecovered channel 110 to the remaining good channels 110 therebyre-establishing a fully functional interface across all channels 110 ofthe virtual channel 111 of FIG. 1.

To support timing alignment issues that may otherwise be handled usingdeskewing logic, the MCU 106 and the memory buffer chip 202 may supportthe use of tags. Command completion and data destination routinginformation can be stored in a tag directory 424 which is accessed usinga received tag. Mechanisms for error recovery, including retrying ofread or write commands, may be implemented in the memory buffer chips202 for each individual channel 110. Each command that is issued by theMCU 106 to the memory buffer chips 202 may be assigned a command tag inthe MCU 106, and the assigned command tag sent with the command to thememory buffer chips 202 in the various channels 110. The variouschannels 110 send back response tags that comprise data tags or donetags. Data tags corresponding to the assigned command tag are returnedfrom the buffer chip in each channel to correlate read data that isreturned from the various channels 110 to an original read command. Donetags corresponding to the assigned command tag are also returned fromthe memory buffer chip 202 in each channel 110 to indicate read or writecommand completion.

The tag directory 424, also associated with tag tables which can includea data tag table and a done tag table, may be maintained in the MCU 106to record and check the returned data and done tags. It is determinedbased on the tag tables when all of the currently functioning channelsin communication with the MCU 106 return the tags corresponding to aparticular command. For data tags corresponding to a read command, theread data is considered available for delivery to the cache subsystem118 of FIG. 1 when a data tag corresponding to the read command isdetermined to have been received from each of the currently functioningchannels 110. For done tags corresponding to a read or write command,the read or write is indicated as complete from a memory control unitand system perspective when a done tag corresponding to the read orwrite command is determined to have been received from each of thecurrently functioning channels 110. The tag checking mechanism in theMCU 106 may account for a permanently failed channel 110 by removingthat channel 110 from a list of channels 110 to check in the tag tables.No read or write commands need to be retained in the MCU 106 forretrying commands, freeing up queuing resources within the MCU 106.

Timing and signal adjustments to support high-speed synchronouscommunications are also managed at the interface level for the channels110. FIG. 5 depicts an example of channel 110 and interfaces 214 and 216in greater detail in accordance with an embodiment. As previouslydescribed in reference to FIG. 1, each channel 110 includes a downstreambus 114 and an upstream bus 116. The downstream bus 114 includesmultiple downstream lanes 502, where each lane 502 can be a differentialserial signal path to establish communication between a driver buffer504 of interface 216 and a receiver buffer 506 of interface 214.Similarly, the upstream bus 116 includes multiple upstream lanes 512,where each lane 512 can be a differential serial signal path toestablish communication between a driver buffer 514 of interface 214 anda receiver buffer 516 of interface 216. In an exemplary embodiment,groups 508 of four bits are transmitted serially on each of the activetransmitting lanes 502 per frame, and groups 510 of four bits aretransmitted serially on each of the active transmitting lanes 512 perframe; however, other group sizes can be supported. The lanes 502 and512 can be general data lanes, clock lanes, spare lanes, or other lanetypes, where a general data lane may send command, address, tag, framecontrol or data bits.

In interface 216, commands and/or data are stored in a transmitfirst-in-first-out (FIFO) buffer 518 to transmit as frames 520. Theframes 520 are serialized by serializer 522 and transmitted by thedriver buffers 504 as groups 508 of serial data on the lanes 502 tointerface 214. In interface 214, serial data received at receiverbuffers 506 are deserialized by deserializer 524 and captured in areceive FIFO buffer 526, where received frames 528 can be analyzed andreconstructed. When sending data from interface 214 back to interface216, frames 530 to be transmitted are stored in a transmit FIFO buffer532 of the interface 214, serialized by serializer 534, and transmittedby the driver buffers 514 as groups 510 of serial data on the lanes 512to interface 216. In interface 216, serial data received at receiverbuffers 516 are deserialized by deserializer 536 and captured in areceive FIFO buffer 538, where received frames 540 can be analyzed andreconstructed.

The interfaces 214 and 216 may each include respective instances oftraining logic 544 and 546 to configure the interfaces 214 and 216. Thetraining logic 544 and 546 train both the downstream bus 114 and theupstream bus 116 to properly align a source synchronous clock totransmissions on the lanes 502 and 512. The training logic 544 and 546also establish a sufficient data eye to ensure successful data capture.Further details are described in reference to process 600 of FIG. 6.

FIG. 6 depicts a process 600 for providing synchronous operation in amemory subsystem in accordance with an embodiment. In order toaccomplish high availability fully synchronous memory operation acrossall multiple channels 110, an initialization and synchronization processis employed across the channels 110. The process 600 is described inreference to elements of FIGS. 1-5.

At block 602, the lanes 502 and 512 of each channel 110 are initializedand calibrated. The training logic 544 and 546 can perform impedancecalibration on the driver buffers 504 and 514. The training logic 544and 546 may also perform static offset calibration of the receiverbuffers 506 and 516 and/or sampling latches (not depicted) followed by awire test to detect permanent defects in the transmission media ofchannel 110. Wire testing may be performed by sending a slow patternthat checks wire continuity of both sides of the clock and data lanedifferential pairs for the lanes 502 and 512. The wire testing mayinclude driving a simple repeating pattern to set a phase rotatorsampling point, synchronize the serializer 522 with the deserializer 524and the serializer 534 with the deserializer 536, and perform lane-baseddeskewing. Data eye optimization may also be performed by sending a morecomplex training pattern that also acts as a functional data scramblingpattern.

Training logic 544 and 546 can use complex training patterns to optimizevarious parameters such as a final receiver offset, a final receivergain, peaking amplitude, decision feedback equalization, final phaserotator adjustment, final offset calibration, scrambler and descramblersynchronization, and load-to-unload delay adjustments for FIFOs 518,526, 532, and 538.

Upon detecting any non-functional lanes in the lanes 502 and 512, adynamic sparing process is invoked to replace the non-functional/brokenlane with an available spare lane of the corresponding downstream bus114 or upstream bus 116. A final adjustment may be made to read dataFIFO unload pointers of the receive FIFO buffers 526 and 538 to ensuresufficient timing margin.

At block 604, a frame transmission protocol is established based on acalculated frame round trip latency. Once a channel 110 is capable ofreliably transmitting frames in both directions, a reference startingpoint is established for decoding frames. To establish synchronizationwith a common reference between the nest clock 405 and the master clock408, a frame lock sequence is performed by the training logic 546 and544. The training logic 546 may initiate the frame lock sequence bysending a frame including a fixed pattern, such as all ones, to thetraining logic 544 on the downstream bus 114. The training logic 544locks on to the fixed pattern frame received on the downstream bus 114.The training logic 544 then sends the fixed pattern frame to thetraining logic 546 on the upstream bus 116. The training logic 546 lockson to the fixed pattern frame received on the upstream bus 116. Thetraining logic 546 and 544 continuously generate the frame beats. Uponcompletion of the frame lock sequence, the detected frame startreference point is used as an alignment marker for all subsequentinternal clock domains.

A positive acknowledgement frame protocol may be used where the traininglogic 544 and 546 acknowledge receipt of every frame back to thetransmitting side. This can be accomplished through the use ofsequential transaction identifiers assigned to every transmitted frame.In order for the sending side to accurately predict the returningacknowledgment, another training sequence referred to as frame roundtrip latency (FRTL) can be performed to account for the propagationdelay in the transmission medium of the channel 110.

In an exemplary embodiment, the training logic 546 issues a null packetdownstream and starts a downstream frame timer. The training logic 544responds with an upstream acknowledge frame and simultaneously starts anupstream round-trip timer. The training logic 546 sets a downstreamround-trip latency value, when the first upstream acknowledge frame isreceived from the training logic 544. The training logic 546 sends adownstream acknowledge frame on the downstream bus 114 in response tothe upstream acknowledge frame from the training logic 544. The traininglogic 544 sets an upstream round-trip delay value when the downstreamacknowledge frame is detected. The training logic 544 issues a secondupstream acknowledge frame to close the loop. At this time the traininglogic 544 goes into a channel interlock state. The training logic 544starts to issue idle frames until a positive acknowledgement is receivedfor the first idle frame transmitted by the training logic 544. Thetraining logic 546 detects the second upstream acknowledge frame andenters into a channel interlock state. The training logic 546 starts toissue idle frames until a positive acknowledgement is received for thefirst idle frame transmitted by the training logic 546. Upon receipt ofthe positive acknowledgement, the training logic 546 completes channelinterlock and normal traffic is allowed to flow through the channel 110.

At block 606, a common synchronization reference is established formultiple memory buffer chips 202 a-202 n. In the case of a fullysynchronous multi-channel structure, a relative synchronization point isestablished to ensure that operations initiated from the CP system 102are executed in the same manner on the memory buffer chips 202 a-202 n,even when the memory buffer chips 202 a-202 n are also generating theirown autonomous refresh and calibration operations. Synchronization canbe accomplished by locking into a fixed frequency ratio between the nestand memory domains 220 and 224 within each memory buffer chip 202. Inexemplary embodiments, the PLLs 404 and 406 from both the nest andmemory domains 220 and 224 are interlocked such that they have a fixedrepeating relationship. This ensures both domains have a same-edgealigned boundary (e.g., rising edge aligned) at repeated intervals,which is also aligned to underlying clocks used for the high speedsource synchronous interface 214 as well as frame decode and executionlogic of the MBU 218. A common rising edge across all the underlyingclock domains is referred to as the alignment or “golden” referencecycle.

Multi-channel operational synchronization is achieved by using thealignment reference cycle to govern all execution and arbitrationdecisions within the memory buffer chips 202 a-202 n. Since all of thememory buffer chips 202 a-202 n in the same virtual channel 111 have thesame relative alignment reference cycle, all of their queues andarbiters (not depicted) remain logically in lock step. This results inthe same order of operations across all of the channels 110. Even thoughthe channels 110 can have inherent physical skew, and each memory bufferchip 202 performs a given operation at different absolute times withrespect to the other memory buffer chips 202, the common alignmentreference cycle provides an opportunity for channel operations totransit the boundary layer 222 between the nest and memory domains 220and 224 with guaranteed timing closure and equivalent arbitration amonginternally generated refresh and calibration operations.

As previously described in reference to FIG. 4, each memory buffer chip202 includes two discrete PLLs, PLL 404 and PLL 406, for driving theunderlying clocks 405 and 407 of the nest and memory domains 220 and224. When operating in asynchronous mode, each PLL 404 and 406 hasdisparate reference clock inputs 408 and 414 with no inherent phaserelationship to one another. However, when running in synchronous mode,the memory PLL 406 becomes a slave to the nest PLL 404 with the modeselector 420 taking over the role of providing a reference clock 418 tothe memory PLL 406 such that memory domain clocks 407 align to thecommon alignment reference point. A common external reference clock, themaster clock 408, may be distributed to the nest PLLs 404 of all memorybuffer chips 202 a-202 n in the same virtual channel 111. The PLL 404can be configured into an external feedback mode to ensure that all PLLs404 align their output nest clocks 405 to a common memory sub-systemreference point. This common point is used by dedicated sync logic todrive the appropriate reference clock 418 based on PLL 404 output 416into the memory domain PLL 406 and achieve a lock onto the targetalignment cycle (i.e., the “golden” cycle).

FIG. 7 depicts a process 700 for establishing alignment between the nestand memory domains 220 and 224 in a memory subsystem 112 accordance withan embodiment. The process 700 is described in reference to elements ofFIGS. 1-6. The process 700 establishes an alignment or “golden” cyclefirst in the nest domain 220 followed by the memory domain 224. Allinternal counters and timers of a memory buffer chip 202 are aligned tothe alignment cycle by process 700.

At block 702, the nest domain clocks 405 are aligned with a frame startsignal from a previous frame lock of block 604. The nest domain 220 canuse multiple clock frequencies for the nest domain clocks 405, forexample, to save power. A frame start may be defined using a higherspeed clock, and as such, the possibility exists that the frame startcould fall in a later phase of a slower-speed nest domain clock 405.This would create a situation where frame decoding would not beperformed on an alignment cycle. In order to avoid this, the frame startsignal may be delayed by one or more cycles, if necessary, such that italways aligns with the slower-speed nest domain clock 405, thereby edgealigning the frame start with the nest domain clocks 405. Clockalignment for the nest domain clocks 405 can be managed by the PLL 404and/or additional circuitry (not depicted). At block 704, the memorydomain clocks 407 are turned off and the memory domain PLL 406 is placedinto bypass mode.

At block 706, the MCS 108 issues a super synchronize (“SuperSync”)command using a normal frame protocol to all memory buffer chips 202a-202 n. The MCS 108 may employ a modulo counter matching an establishedfrequency ratio such that it will only issue any type of synchronizationcommand at a fixed period. This establishes the master reference pointfor the entire memory subsystem 112 from the MCS 108 perspective. Eventhough the SuperSync command can arrive at the memory buffer chips 202a-202 n at different absolute times, each memory buffer chip 202 can usea nest cycle upon which this command is decoded as an internal alignmentcycle. Since skew among the memory buffer chips 202 a-202 n is fixed,the alignment cycle on each of the memory buffer chips 202 a-202 n willhave the same fixed skew. This skew translates into a fixed operationalskew under error free conditions.

At block 708, sync logic of the memory buffer chip 202, which may bepart of the mode selector 420, uses the SuperSync decode as a referenceto trigger realignment of the reference clock 418 that drives the memorydomain PLL 406. The SuperSync decode is translated into a one cyclepulse signal 494, synchronous with the nest domain clock 405 that resetsto zero a modulo counter 496 in the FSYNC block 492. The period of thiscounter 496 within the FSYNC block 492 is set to be the least commonmultiple of all memory and nest clock frequencies with the rising edgemarking the sync-point corresponding to the reference point previouslyestablished by the MCS 108. The rising edge of FSYNC clock 416 becomesthe reference clock of PLL 406 to create the memory domain clocks. Bybringing the lower-frequency output of PLL 406 back into the externalfeedback port, the nest clock 405 and memory clock 407 all have a commonclock edge aligned to the master reference point. Thus, the FSYNC block492 provides synchronous clock alignment logic.

At block 710, the memory domain PLL 406 is taken out of bypass mode inorder to lock into the new reference clock 418 based on the output 416of the PLL 404 rather than reference clock 414. At block 712, the memorydomain clocks 407 are turned back on. The memory domain clocks 407 arenow edge aligned to the same alignment reference cycle as the nestdomain clocks 405.

At block 714, a regular subsequent sync command is sent by the MCS 108on the alignment cycle. This sync command may be used to reset thevarious counters, timers and MISRs 226 that govern internal memoryoperation command generation, execution and arbitration. By performing areset on the alignment cycle, all of the memory buffer chips 202 a-202 nstart their respective internal timers and counters with the samelogical reference point. If an arbiter on one memory buffer chip 202identifies a request from both a processor initiated memory operationand an internally initiated command on a particular alignment cycle, thecorresponding arbiter on the remaining memory buffer chips 202 will alsosee the same requests on the same relative alignment cycle. Thus, allmemory buffer chips 202 a-202 n will make the same arbitration decisionsand maintain the same order of operations.

Embodiments may provide internally generated commands at memory bufferchip 202 to include DRAM refresh commands, DDR calibration operations,dynamic power management, error recovery, memory diagnostics, and thelike. Anytime one of these operations is needed, it must cross into thenest domain 220 and go through the same arbitration as synchronousoperations initiated by the MCS 108. Arbitration is performed on thegolden cycle to ensure all the memory buffer chips 202 observe the samearbitration queues and generate the same result. The result isdispatched across boundary layer 222 on the golden cycle which ensurestiming and process variations in each memory buffer chip 202 isnullified.

Under normal error free conditions, the order of operations will bemaintained across all of the memory buffer chips 202 a-202 n. However,there are situations where one channel 110 can get out of sync with theother channels 110. One such occurrence is the presence of intermittenttransmission errors on one or more of the interfaces 214 and 216.Exemplary embodiments include a hardware based recovery mechanism whereall frames transmitted on a channel 110 are kept in a replay buffer fora prescribed period of time. This time covers a window long enough toguarantee that the frame has arrived at the receiving side, has beenchecked for errors, and a positive acknowledgement indicating error freetransmission has been returned to the sender. Once this is confirmed,the frame is retired from the replay buffer. However, in the case of anerroneous transmission, the frame is automatically retransmitted, orreplayed, along with a number of subsequent frames in case the error wasa one-time event. In many cases, the replay is sufficient and normaloperation can resume. In certain cases, the transmission medium of thechannel 110 has become corrupted to the point that a dynamic repair isinstituted to replace a defective lane with a spare lane from lanes 502or 512. Upon completion of the repair procedure, the replay of theoriginal frames is sent and again normal operation can resume.

Another less common occurrence can be an on-chip disturbance manifestingas a latch upset which results in an internal error within the memorybuffer chip 202. This can lead to a situation where one memory bufferchip 202 executes its operations differently from the remaining memorybuffer chips 202. Although the memory system 100 continues to operatecorrectly, there can be significant performance degradation if thechannels 110 do not operate in step with each other. In exemplaryembodiments, the MISRs 226 monitor for and detect such a situation. TheMISRs 226 receive inputs derived from key timers and counters thatgovern the synchronous operation of the memory buffer chip 202, such asrefresh starts, DDR calibration timers, power throttling, and the like.The inputs to the MISRs 226 are received as a combination of bits thatcollectively form a signature. One or more of the bits of the MISRs 226are continually transmitted as part of an upstream frame payload to theMCU 106, which monitors the bits received from the MISRs 226 of thememory buffer chips 202 a-202 n. The presence of physical skew betweenthe channels 110 results in the bits from the MISRs 226 arriving atdifferent absolute times across the channels 110. Therefore, a learningprocess is incorporated to calibrate checking of the MISRs 226 to thewire delays in the channels 110.

In exemplary embodiments, MISR detection in the MCU 106 incorporates twodistinct aspects in order to monitor the synchronicity of the channels110. First, the MCU 106 monitors the MISR bits received on the upstreambus 116 from each of the memory buffer chips 202 a-202 n and anydifference seen in the MISR bit stream indicates an out-of-synccondition. Although this does not pose any risk of a data integrityissue, it can negatively impact performance, as the MCU 106 may incuradditional latency waiting for an entire cache line access to completeacross the channels 110. Another aspect is monitoring transactionsequence identifiers (i.e., tags) associated with each memory operationand comparing associated “data” tags or “done” tags as the operationscomplete. Once again, skew of the channels 110 is taken into account inorder to perform an accurate comparison. In one example, this skew canmanifest in as many as 30 cycles of difference between the fastest andslowest channel 110. If the tags are 7-bits wide, with five channels110, and a maximum 30-cycle difference across channels 110, this wouldtypically require 5×7×30=1050 latches to perform a simplistic compare.There may be some cases that equate to about 40 bit-times which is about4 cycles of deskew after aligning to a frame. To further reduce thenumber of latches, a MISR can be incorporated within the MCU 106 toencode the tag into a bit stream, which is then pipelined to eliminatethe skew. By comparing the output of the MISR of the MCU 106 across allof the channels 110, a detected difference indicates an out-of-orderprocessing condition.

In either of these situations, the afflicted channel 110 can at leasttemporarily operate out of sync or out of order with respect to theother channels 110. Continuous availability of the memory subsystem 112may be provided through various recovery and self-healing mechanisms.Data tags can be used such that in the event of an out-of-order orout-of-sync condition, the MCU 106 continues to function. Each readcommand may include an associated data tag that allows the MCS 108 tohandle data transfers received from different channels 110 at differenttimes or even in different order. This allows proper functioning even insituations when the channels 110 go out of sync.

For out-of-sync conditions, a group of hierarchical MISRs 226 can beused accumulate a signature for any sync-related event. Examples ofsync-related events include a memory refresh start, a periodic driver(ZQ) calibration start, periodic memory calibration start, powermanagement window start, and other events that run off a synchronizedcounter. One or more bits from calibration timers, refresh timers, andthe like can serve as inputs to the MISRs 226 to provide a time varyingsignature which may assist in verifying cross-channel synchronization atthe MCU 106. Hierarchical MISRs 226 can be inserted wherever there is aneed for speed matching of data. For example, speed matching may beneeded between MBA 212 a and the MBU 218, between the MBA 212 b and theMBU 218, between the MBU 218 and the upstream bus 116, and between theinterfaces 216 a-216 n and the MCS 108.

For out-of-order conditions, staging each of the tags received in framesfrom each channel 110 can be used to deskew the wire delays and comparethem. A MISR per channel 110 can be used to create a signature bitstream from the tags received at the MCU 106 and performtag/signature-based deskewing rather than hardware latch-baseddeskewing. Based on the previous example of 7-bit wide tags, with fivechannels 110, and a maximum 30-cycle difference across channels 110, theuse of MISRs reduces the 1050 latches to about 7×5+30×5=185 latches,plus the additional support latches.

To minimize performance impacts, the MCS 108 tries to keep all channels110 in lockstep, which implies that all commands are executed in thesame order. When read commands are executed, an associated data tag isused to determine which data correspond to which command. This approachalso allows the commands to be reordered based on resource availabilityor timing dependencies and to get better performance. Commands may bereordered while keeping all channels 110 in lockstep such that thereordering is the same across different channels 110. In this case, tagscan be used to match the data to the requester of the data from memoryregardless of the fact that the command order changed while the datarequest was processed.

Marking a channel 110 in error may be performed when transfers havealready started and to wait for recovery for cases where transfers havenot yet occurred. Data blocks from the memory subsystem 112 can bedelivered to the cache subsystem interface 122 of FIG. 1 as soon as datais available without waiting for complete data error detection. Thisdesign implementation is based on the assumption that channel errors arerare. Data can be sent across clock domains from the MCS 108 to thecache subsystem interface 122 asynchronously as soon as it is availablefrom all channels 110 but before data error detection is complete forall frames. If a data error is detected after the data block transferhas begun, an indication is sent from the MCS 108 to the cache subsysteminterface 122, for instance, on a separate asynchronous interface, tointercept the data block transfer in progress and complete the transferusing redundant channel information. Timing requirements are enforced toensure that the interception occurs in time to prevent propagation ofcorrupt data to the cache subsystem 118 of FIG. 1. A programmablecount-down counter may be employed to enforce the timing requirements.

If the data error is detected before the block data transfer has begunto the cache subsystem 118, the transfer is stalled until all frameshave been checked for any data errors. Assuming errors are infrequent,the performance impact is minimal. This reduces the use of channelredundancy and may result in avoidance of possible uncorrectable errorsin the presence of previously existing errors in the DRAM devices 204.

The MCU 106 may also include configurable delay functions on aper-command type or destination basis to delay data block transfer toupstream elements, such as caches, until data error detection iscompleted for the block. Command or destination information is availablefor making such selections as inputs to the tag directory. This canselectively increase system reliability and simplify error handling,while minimizing performance impacts.

To support other synchronization issues, the MCU 106 can re-establishsynchronization across multiple channels 110 in the event of a channelfailure without having control of an underlying recovery mechanism usedon the failed channel. A programmable quiesce sequence incrementallyattempts to restore channel synchronization by stopping stores and otherdownstream commands over a programmable time interval. The quiescesequence may wait for completion indications from the memory bufferchips 202 a-202 n and inject synchronization commands across allchannels 110 to reset underlying counters, timers, MISRs 226, and othertime-sensitive circuitry to the alignment reference cycle. If a failedchannel 110 remains out of synchronization, the quiesce sequence can beretried under programmatic control. In many circumstances, theunderlying root cause of the disturbance can be self healed, therebyresulting in the previously failed channel 110 being reactivated andresynchronized with the remaining channels 110. Under extreme errorconditions the quiesce and recovery sequence fails to restore the failedchannel 110, and the failed channel 110 is permanently taken off line.In a RAIM architecture that includes five channels 110, the failure ofone channel 110 permits the remaining four channels 110 to operate witha reduced level of protection.

FIG. 8 depicts an example timing diagram 800 of synchronizing a memorysubsystem in accordance with an embodiment. The timing diagram 800includes timing for a number of signals of the memory buffer chip 202.In the example of FIG. 8, two of the nest domain clocks 405 of FIG. 4are depicted as a higher-speed nest domain clock frequency 802 and alower-speed nest domain clock frequency 804. Two of the memory domainclocks 407 of FIG. 4 are depicted in FIG. 8 as a higher-speed memorydomain clock frequency 806 and a lower-speed memory domain clockfrequency 808. The timing diagram 800 also depicts example timing for anest domain pipeline 810, a boundary layer 812, a reference counter 814,a memory queue 816, and a DDR interface 818 of a DDR port 210. In anembodiment, the higher-speed nest domain clock frequency 802 is about2.4 GHz, the lower-speed nest domain clock frequency 804 is about 1.2GHz, the higher-speed memory domain clock frequency 806 is about 1.6,GHz and the lower-speed memory domain clock frequency 808 is about 0.8GHz.

A repeating pattern of clock cycles is depicted in FIG. 8 as a sequenceof cycles “B”, “C”, “A” for the lower-speed nest domain clock frequency804. Cycle A represents an alignment cycle, where other clocks andtimers in the memory buffer chip 202 are reset to align with a risingedge of the alignment cycle A. Upon receiving a SuperSync command, thehigher and lower-speed memory domain clock frequencies 806 and 808 stopand restart based on a sync point that results in alignment after aclock sync window 820. Once alignment is achieved, the alignment cycleA, also referred to as a “golden” cycle, serves as a common logicalreference for all memory buffer chips 202 a-202 n in the same virtualchannel 111. Commands and data only cross the boundary layer 222 on thealignment cycle. A regular sync command can be used to reset countersand timers within each of the memory buffer chips 202 a-202 n such thatall counting is referenced to the alignment cycle.

In FIG. 8 at clock edge 822, the higher and lower-speed nest domainclock frequencies 802 and 804, the higher and lower-speed memory domainclock frequencies 806 and 808, and the nest domain pipeline 810 are allaligned. A sync command in the nest domain pipeline 810 is passed to theboundary layer 812 at clock edge 824 of the higher-speed memory domainclock frequency 806. At clock edge 826 of cycle B, a read command isreceived in the nest domain pipeline 810. At clock edge 828 of thehigher-speed memory domain clock frequency 806, the read command ispassed to the boundary layer 812, the reference counter 814 startscounting a zero, and the sync command is passed to the memory queue 816.At clock edge 830 of the higher-speed memory domain clock frequency 806,the reference counter 814 increments to one, the read command is passedto the memory queue 816 and the DDR interface 818. At clock edge 832 ofthe higher-speed memory domain clock frequency 806 which aligns with analignment cycle A, the reference counter 814 increments to two, and arefresh command is queued in the memory queue 816. Alignment is achievedbetween clocks and signals of the nest domain 220 and the memory domain224 for sending commands and data across the boundary layer 222 of FIG.2.

FIG. 9A illustrates an embodiment of the MCU 106 of FIG. 1 in greaterdetail. In MCU 106, tag allocation logic 901 receives read and writecommands from the cache subsystem 118 of FIG. 1 and assigns a commandtag to each read and write command. The available command tags may benumbered from 0 to 31 in some embodiments; in such an embodiment, 0-23may be reserved for read commands, and 24-31 may be reserved for writecommands. Frame generation logic 902 sends read and write commands, andcorresponding command tags, to memory buffer chips 202 of FIGS. 2-4corresponding to a plurality of channels 110 in the virtual channel 111of FIG. 1 (for example, five memory buffer chips, each corresponding toa single channel 110 of FIGS. 1-4) via downstream buses 903(corresponding to downstream bus 114 of FIG. 1), and read data and tags(including data and done tags) are returned from the memory buffer chips202 on all channels 110 to memory control unit interfaces 905 viaupstream buses 904 (corresponding to upstream bus 116 of FIG. 1). Memorycontrol unit interfaces 905 may comprise a separate interface 216 ofFIGS. 2-4 for each of the channels 110. The memory control unitinterfaces 905 separate read data from tags, and sends read data on readdata buses 906 to buffer control blocks 908A-E.

The MCU 106 includes a respective buffer control block 908A-E for eachchannel 110. These buffer control blocks 908A-E store data that arereturned on read data buses 906 from memory control unit interfaces 905until the data may be sent to the cache subsystem 118 of FIG. 1 via databuffer read control and dataflow logic 912. Data buffer read control anddataflow logic 912 may be part of the cache subsystem interface 122 ofFIG. 1 and configured to communicate with the cache subsystem 118 ofFIG. 1. Accordingly, the data buffer read control and dataflow logic 912of the cache subsystem interface 122 can be part of the MCU 106 or beseparate but interfaced to the MCU 106. An embodiment may have the databuffer read control and dataflow logic 912 as part of a system domain916, whereas the other blocks shown as part of the MCU 106 may be partof a memory controller nest domain 918. In this embodiment, data buffersin buffer control blocks 908A-E are custom arrays that support writingstore data into the buffers which is performed in the memory controllernest domain 918 and reading data out of the same buffers which isperformed in the system domain 916.

Memory control unit interfaces 905 send the tag information via tagbuses 907 to both the buffer control blocks 908A-E and channel controlblock 909. Channel control block 909 tracks received tags using the datatag table 910 and done tag table 911, each of which are shown in furtherdetail in FIG. 9B. Data tag table 910 and done tag table 911 areassociated with the tag directory 424 of FIG. 4, where each of thetables 910 and 911 includes a plurality of rows, each row corresponds toa command tag, and each row includes a respective bit corresponding toeach of the channels 110 in communication with the MCU 106 (e.g.,channels 0 through 4). In some embodiments, the rows in data tag table910 may be numbered from 0 to 23, corresponding to the command tagsreserved for read commands, and the rows in done tag table 911 may benumbered from 0 to 31, corresponding to all available command tags. Thechannel control block 909 also indicates to tag allocation logic 901when a command tag may be reused, and indicates to data buffer readcontrol and dataflow logic 912 that a command is completed. FIGS. 9A-Bare shown for illustrative purposes only; for example, any appropriatenumber of command tags may be available in a tag allocation logic 901,and a data tag table 910 and done tag table 911 may each include anyappropriate number of rows. Further, any appropriate number of channels110, with associated buffer control blocks 908A-E and respective bits inthe data tag table 910 and done tag table 911, may be in communicationwith the MCU 106.

In the example of FIG. 9A, there are five memory channels 110, wherefour channels 110 provide data and ECC symbols, and the fifth channel110 provides redundant data used for channel recovery. In an embodiment,a data block includes 256 bytes of data which is subdivided into four64-byte sections otherwise known as quarter-lines (QL). Each QL hasassociated with it 8 bytes of ECC symbols. The resulting 72 bytes aredistributed across four of the five channels 110 using an 18-byteinterleave. The fifth channel 110 stores channel redundancy (RAIM)information. This QL sectioning allows for recovery of any QL of data inthe event of DRAM failures or an entire channel 110 failure.

Each QL of data and check bytes (or corresponding channel redundancyinformation in the case of the fifth channel 110) is contained in aframe that is transmitted across a channel 110. This frame contains CRCinformation to detect errors with the data in the frame along withattributes contained in the frame and associated with the data. One ofthese attributes is a data tag which is used to match incoming framedata with previously sent fetch commands to memory. Another attribute isa done tag which is used to indicate command completion in a memorybuffer chip 202. Each fetch command is directly associated with a256-byte data block fetch through an assigned data tag. In this example,a 256-byte fetch requires four QL's or frames per channel. These fourframes are sent consecutively by each channel 110 assuming no errors arepresent.

Data arrives independently from each of the five channels 110 and arematched up with data from other channels 110 using the previouslymentioned data tag. The channel control block 909 tracks the receivedtags using the data tag table 910 and the done tag table 911. Thechannel control block 909 sets a bit in the data tag table 910 based onreceiving a data tag, and sets a bit in the done tag table 911 based onreceiving a done tag. Embodiments of data tag table 910 and done tagtable 911 are shown in further detail in FIG. 9B. Each row in the datatag table 910 corresponds to a command tag associated with a single readcommand (numbered from 0 to 23), and each row has an entry comprising abit for each of the five channels 110. As shown in data tag table 910 ofFIG. 9B, the first row corresponds to data tag 0 for all channels 110,and the last row corresponds to data tag 23 for all channels 110. Eachrow in the done tag table 911 corresponds to a command tag associatedwith a single read or write command (numbered from 0 to 31), and eachrow has an entry comprising a bit for each of the five channels 110. Asshown in done tag table 911 of FIG. 9B, the first row corresponds todone tag 0 for all channels, and the last row corresponds to done tag 31for all channels 110. When a data or done tag is received from anindividual channel 110, the bit for that channel 110 in the rowcorresponding to the tag of the data tag table 910 or done tag table 911is set to indicate that that particular data tag or done tag has beenreceived. The data tags are also used as write pointers to bufferlocations in buffer control blocks 908A-E. Each of buffer control blocks908A-E holds read data received from the buffer control block'srespective channel on read data buses 906. In some embodiments, thebuffer locations in buffer control blocks 908A-E may be numbered from 0to 23, corresponding to the numbers of the command tags that arereserved for read commands. Read data received on a particular channelare loaded into the location in the channel's buffer control block908A-E that is indicated by the data tag that is received with the readdata.

A controller 914, which may be part of the channel control block 909,controls the transfer of data through the buffer control blocks 908A-Eto the data buffer read control and dataflow logic 912. The data bufferread control and dataflow logic 912 can read the buffer control blocks908A-E asynchronously relative to the memory control unit interfaces 905populating the buffer control blocks 908A-E with data. The data bufferread control and dataflow logic 912 operates in the system domain 916,while the memory control unit interfaces 905 operate in the memorycontroller nest domain 918, where the system domain 916 and the memorycontroller nest domain 918 are asynchronous clock domains that may havea variable frequency relationship between domains. Accordingly, thebuffer control blocks 908A-E form an asynchronous boundary layer betweenthe system domain 916 and the memory controller nest domain 918. In anexemplary embodiment, the controller 914 is in the memory controllernest domain 918 and sends signals to the data buffer read control anddataflow logic 912 via an asynchronous interface 920.

The MCU 106 also includes an out-of-sync/out-of-order (OOS/OOO) detector922 that may be incorporated in the memory control unit interfaces 905.The OOS/OOO detector 922 compares values received across multiplechannels 110 for out-of-synchronization conditions and/or out-of-orderconditions. Data tags and done tags received in frames on the channels110 can be used in combination with data from the MISRs 226 of FIGS. 2-4and other values to detect and compare synchronization events across thechannels 110. Tag-based detection allows for a greater degree offlexibility to maintain synchronization and data order as compared tostricter cycle-based synchronization. A resynchronization process may beinitiated upon determining that at least one of the channels 110 issubstantially out of synchronization or out of order. The OOS/OOOdetector 922 and other elements of the MCU 106 can be implemented usinga processing circuit that can include application specific integratedcircuitry, programmable logic, or other underlying technologies known inthe art.

The controller 914 can control reestablishing synchronization in the MCU106. In an exemplary embodiment, the controller 914 interfaces with theOOS/OOO detector 922, a number of counters 924, timers 926, and a replaysystem 928 to reestablish synchronization. The counters 924 and timers926 can be used for process control flow to reestablish synchronization.In an exemplary embodiment, the replay system 928 is a separate portionof the CP 102 of FIG. 1 and is not directly controlled by the MCU 106.An error condition on one of the channels 110, such as a loss of one ormore data bits within a frame, can be detected by a CRC mismatch, whichtriggers a channel error recovery or replay sequence by the replaysystem 928. A similar replay system also exists on the memory bufferchips 202. The replay system 928 can provide status information, such asa replay-active signal 930 to the controller 914 of the MCU 106. Theflow of both downstream store data and commands, and upstream fetch datais interrupted as a result of a replay sequence on an individual channel110. The replay sequence causes the faulty channel 110 to go out ofsynchronization with the other four channels 110. The MCU 106 can handlea channel 110 being out of synchronization for a period of time, butthere is an impact to system performance if this out of synchronizationcondition is maintained for a long period. The MCU 106 is configured torestore synchronization in a timely fashion after a channel error eventso that the system performance impact is minimized.

FIGS. 10A, 10B, and 10C depict a process 1000 for reestablishingsynchronization across multiple memory channels 110 in the memory system100 of FIG. 1 in accordance with an embodiment. The process 1000 can beimplemented by the MCU 106 as described in reference to FIGS. 1-9A.

As is depicted in FIGS. 10A-10C, three stages are implemented underprogrammatic control for progressively increasing the complexity of thequiese sequence to reestablish synchronization in the example memorysystem 100 of FIG. 1. The use of multiple programmable timers 926 ofFIG. 9A, e.g., timers T1-T4, enable tuning the process 1000 for specificsystem characteristics such as error rate, recovery time, asynchronousmemory events and higher-level system time-out mechanisms. Asynchronousevents such as refreshes, DRAM interface calibrations and memorythrottling for power in particular can interfere with re-establishingsynchronization. More specifically, one memory buffer chip 202 of FIGS.2-4 can experience interruptions in command flow due to a replay thatother memory buffer chips 202 on other channels 110 will not experience.It is possible under these conditions that the different memory bufferchips 202 will schedule operations differently and in a different orderdue to interference from asynchronous events. Scheduling of asynchronousevents such a refreshes can also be disturbed on the channel 110 withthe replay, further aggravating the out-of-sync or out-of-orderbehavior. These asynchronous events, although relatively infrequent, areaccounted for in the incremental actions taken by the process 1000.

Stage 1 of the process 1000 employs the simplest sequence for restoringsynchronization by stopping the flow of downstream commands and storedata on all channels 1000 until a first programmable timer T1 expires.In an exemplary embodiment, the timer T1 is programmable between about32 and 1024 nanoseconds. The process 1000 is entered, if enabled by theMCU 106, whenever an out-of-sync condition is detected as at block 1002.An out-of-sync condition can be detected by the OOS/OOO detector 922 ofFIG. 9A as a difference in arrival times of fetch data for an individualfetch command between the five channels 110 that does not match apre-defined value determined during system initialization. At block1004, upon detecting an out-of-sync condition 1002, a check is performedto determine whether a memory buffer chip out-of-sync condition exists,which may be based on the MISRs 226 a and 226 b of FIGS. 2-4. Forexample, the OOS/OOO detector 922 of FIG. 9A can compare values orsummary values of the MISRs 226 a and 226 b of FIGS. 2-4 after adjustingfor skew value differences between the channels 110 to determine whethera memory buffer chip out-of-sync condition exists. If no memory bufferchip out-of-sync condition exists at block 1004, the process 1000advances to block 1006.

A loop count, which may be one of the counters 924 of FIG. 9A, isincremented for each pass through stages 1, 2, or 3 of process 1000, anda programmable register determines how many tries each stage gets inre-establishing synchronization. A decision block exists in each stageto test the loop count, determine if it has exceeded the programmed loopcount, and either repeat the stage if needed or move to the next stage.At block 1006, if a stage 1 loop count is not exceeded, the process 1000advances to block 1008. At block 1008, new traffic is selectivelystopped on the channels 110. One of the counters 924 of FIG. 9A can beused as a skip counter, where stopping of new traffic is performed atblock 1008 unless the skip counter is enabled and not exceeded. The skipcounter is incremented with each pass through block 1008 and is clearedwhen quiesce successfully completes. At block 1010, the first timer T1is decremented if no replay is in progress. The replay-active signal 930from the replay system 928 of FIG. 9A can be used to check whether areplay is in progress. At block 1012, if the first timer T1 has notexpired, the process 1000 loops back to block 1010; otherwise, theprocess 1000 advances to block 1014.

At block 1014, traffic resumes on the channels 110 when the first timerT1 expires and no replays are in progress on any of the channels 110. Atblock 1016, a check is performed to determine whether synchronization ismaintained for a time period defined using a programmable second timerT2. The second timer T2 may be a programmable time window or commandcount. If synchronization is verified as reestablished for the timeperiod defined using the second timer T2, the process 1000 returns toblock 1002 to end the quiesce sequence; otherwise, the process 1000returns to block 1004.

At block 1004, if a memory buffer chip MISR out-of-sync conditionexists, the process 1000 advances to block 1038 in stage 3 of FIG. 10Cas depicted by connector A. At block 1006, if the stage 1 loop count isexceeded, the process 1000 advances to block 1018 in stage 2 of FIG. 10Bas depicted by connector B.

Stage 2 is similar to stage 1 except that additional checks are addedprior to starting the timer T1 and waiting for it to expire. At block1018, if a stage 2 loop count is exceeded, then a check is performed atblock 1019 to determine whether stage 3 of FIG. 10C is configured. Ifstage 3 is configured, then the process 1000 advances to stage 3 of FIG.10C as depicted by connector D; otherwise, a failure is declared atblock 1020. At block 1018, if the stage 2 loop count is not exceeded,the process 1000 advances to block 1022. At block 1022, new traffic isstopped on the channels 110. At block 1024, the process 1000 waits foroutstanding traffic to complete on the channels 110. The check foroutstanding traffic may be performed by reviewing a list of pendingfetches and stores. Pending fetches and stores are removed from the listwhen done tags respectively are received on all channels 110 for theseoperations. At block 1026, the process 1000 waits for a write reorderqueue (WRQ) empty status indicator from the memory buffer chips 202 ofFIGS. 2-4, if enabled. The WRQ empty status indicator may be used toindicate that no further memory writes are queued at the memory bufferchips 202.

At block 1028, the first timer T1 is decremented if no replay is inprogress. At block 1030, if the first timer T1 has not expired, theprocess 1000 loops back to block 1028; otherwise, the process 1000advances to block 1032. At block 1032, traffic resumes on the channels110. At block 1034, a check is performed to determine whethersynchronization is verified as reestablished for a time period definedusing the second timer T2. If synchronization is not maintained for thetime period defined using the second timer T2, the process 1000 advancesto block 1036; otherwise, the process 1000 returns to block 1002 instage 1 of FIG. 10A as depicted by connector C to end the quiescesequence. At block 1036, if a memory buffer chip out-of-sync conditionexists, the process 1000 advances to block 1038 in stage 3 of FIG. 10Cas depicted by connector D; otherwise, the process 1000 returns to block1018 to repeat stage 2.

Stage 3 adds an additional step of sending a synchronization commanddown all channels 110 after executing the same initial sequence as stage2. At block 1038, if a stage 3 loop count is exceeded, then a failure isdeclared at block 1040; otherwise, the process 1000 advances to block1042. At block 1042, new traffic is stopped on the channels 110. Atblock 1044, the process 1000 waits for outstanding traffic to completeon the channels 110. At block 1046, the process 1000 wait for the WRQempty status indicator from the memory buffer chips 202 of FIGS. 2-4, ifenabled. At block 1048 a synchronization command is sent on the channels110. The synchronization command resets various timers and counterswithin the memory buffer chips 202 such as those associated withrefresh, interface calibration and memory throttling. At block 1050, aprogrammable third timer T3 is used to wait for a guard time afterissuing the synchronization command at block 1048. In an exemplaryembodiment, the timer T3 is programmable between about 32 and 128nanoseconds. The timer T3 is used to test for successful completion ofthe synchronization command across channels 110 without replay duringthe time period defined by timer T3.

At block 1052, a check is performed to determine whether replay wasactive during the guard time. If replay was active during the guardtime, then the process 1000 returns to block 1048 after waiting anamount of time defined using a programmable fourth timer T4 at block1054. In an exemplary embodiment, the timer T4 is programmable betweenabout 16 and 128 microseconds. The timer T4 waits for the effect of thesynchronization command to clear all of the memory buffer chips 202. Ifreplay was not active during the guard time, then the process 1000 waitsan amount of time defined using the fourth timer T4 at block 1056 andcontinues to block 1058. At block 1058, traffic resumes on the channels110. At block 1060, a check is performed to determine whethersynchronization is verified as reestablished for a time period definedusing the second timer T2. If synchronization is not maintained for thetime period defined using the second timer T2, the process 1000 returnsto block 1038; otherwise, the process 1000 returns to block 1002 instage 1 of FIG. 10A as depicted by connector E to end the quiescesequence.

As will be appreciated by one skilled in the art, one or more aspects ofthe present invention may be embodied as a system, method or computerprogram product. Accordingly, one or more aspects of the presentinvention may take the form of an entirely hardware embodiment, anentirely software embodiment (including firmware, resident software,micro-code, etc.) or an embodiment combining software and hardwareaspects that may all generally be referred to herein as a “circuit,”“module” or “system”. Furthermore, one or more aspects of the presentinvention may take the form of a computer program product embodied inone or more computer readable medium(s) having computer readable programcode embodied thereon.

Any combination of one or more computer readable medium(s) may beutilized. The computer readable medium may be a computer readablestorage medium. A computer readable storage medium may be, for example,but not limited to, an electronic, magnetic, optical, electromagnetic,infrared or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. More specific examples (a non-exhaustivelist) of the computer readable storage medium include the following: anelectrical connection having one or more wires, a portable computerdiskette, a hard disk, a random access memory (RAM), a read-only memory(ROM), an erasable programmable read-only memory (EPROM or Flashmemory), an optical fiber, a portable compact disc read-only memory(CD-ROM), an optical storage device, a magnetic storage device, or anysuitable combination of the foregoing. In the context of this document,a computer readable storage medium may be any tangible medium that cancontain or store a program for use by or in connection with aninstruction execution system, apparatus, or device.

Referring now to FIG. 11, in one example, a computer program product1100 includes, for instance, one or more storage media 1102, wherein themedia may be tangible and/or non-transitory, to store computer readableprogram code means or logic 1104 thereon to provide and facilitate oneor more aspects of embodiments described herein.

Program code, when created and stored on a tangible medium (includingbut not limited to electronic memory modules (RAM), flash memory,Compact Discs (CDs), DVDs, Magnetic Tape and the like is often referredto as a “computer program product”. The computer program product mediumis typically readable by a processing circuit preferably in a computersystem for execution by the processing circuit. Such program code may becreated using a compiler or assembler for example, to assembleinstructions, that, when executed perform aspects of the invention.

Technical effects and benefits include reestablishing synchronizationacross multiple memory channels in a memory subsystem. Using amulti-stage approach to reestablishing synchronization allows a fasterand simpler approach to be tried initially before advancing to slowerand more complex sequences of operations for reestablishingsynchronization.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of embodiments. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of embodiments have been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the embodiments in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the embodiments. Theembodiments were chosen and described in order to best explain theprinciples and the practical application, and to enable others ofordinary skill in the art to understand the embodiments with variousmodifications as are suited to the particular use contemplated.

Computer program code for carrying out operations for aspects of theembodiments may be written in any combination of one or more programminglanguages, including an object oriented programming language such asJava, Smalltalk, C++ or the like and conventional procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The program code may execute entirely on the user's computer,partly on the user's computer, as a stand-alone software package, partlyon the user's computer and partly on a remote computer or entirely onthe remote computer or server. In the latter scenario, the remotecomputer may be connected to the user's computer through any type ofnetwork, including a local area network (LAN) or a wide area network(WAN), or the connection may be made to an external computer (forexample, through the Internet using an Internet Service Provider).

Aspects of embodiments are described above with reference to flowchartillustrations and/or schematic diagrams of methods, apparatus (systems)and computer program products according to embodiments. It will beunderstood that each block of the flowchart illustrations and/or blockdiagrams, and combinations of blocks in the flowchart illustrationsand/or block diagrams, can be implemented by computer programinstructions. These computer program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

These computer program instructions may also be stored in a computerreadable medium that can direct a computer, other programmable dataprocessing apparatus, or other devices to function in a particularmanner, such that the instructions stored in the computer readablemedium produce an article of manufacture including instructions whichimplement the function/act specified in the flowchart and/or blockdiagram block or blocks.

The computer program instructions may also be loaded onto a computer,other programmable data processing apparatus, or other devices to causea series of operational steps to be performed on the computer, otherprogrammable apparatus or other devices to produce a computerimplemented process such that the instructions which execute on thecomputer or other programmable apparatus provide processes forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments. In this regard, each block in the flowchart or blockdiagrams may represent a module, segment, or portion of code, whichcomprises one or more executable instructions for implementing thespecified logical function(s). It should also be noted that, in somealternative implementations, the functions noted in the block may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts, or combinations of special purpose hardware andcomputer instructions.

What is claimed is:
 1. A system for reestablishing synchronization across multiple channels in a memory system within a computer, the system comprising: a plurality of channels within the computer each providing communication with a memory buffer chip and a plurality of memory devices; and a memory control unit coupled to the plurality of channels within the computer, the memory control unit configured to perform a method comprising: receiving an out-of-synchronization indication associated with at least one of the channels; performing, by the memory control unit, a first stage of a three stage process of reestablishing synchronization comprising: selectively stopping new traffic on the plurality of channels; waiting for a first time period to expire; resuming traffic on the plurality of channels based on the first time period expiring; verifying that synchronization is reestablished for a second time period; and based on determining that synchronization is not reestablished for the second time period, repeating the first stage of reestablishing synchronization for a number of times before performing a second stage of the three stage process of reestablishing synchronization; performing, by the memory control unit, the second stage of reestablishing synchronization comprising: stopping new traffic on the plurality of channels; waiting for outstanding traffic on the plurality of channels to complete; resuming traffic on the plurality of channels based on the first time period expiring; and verifying that synchronization is reestablished for the second time period; and performing a third stage of the three stage process of reestablishing synchronization based on determining that a memory buffer chip out-of-sync condition exists, the third stage comprising: stopping new traffic on the plurality of channels; waiting for outstanding traffic on the plurality of channels to complete; waiting for a write reorder queue empty status indicator from the memory buffer chips before sending a synchronization command; sending the synchronization command to the memory buffer chips on each of the channels; waiting for a third time period to expire; verifying that a replay did not occur during the third time period, wherein the replay comprises a recovery retransmission sequence from a replay buffer that causes a faulty channel to go out of synchronization with non-faulty instances of the channels; waiting a fourth time period before resuming traffic on the plurality of channels; resuming traffic on the plurality of channels; verifying that synchronization is reestablished for the second time period; and based on determining that synchronization is not reestablished for the second time period, repeating the third stage of reestablishing synchronization for a number of times before declaring a failure.
 2. The system of claim 1, wherein performing the second stage of reestablishing synchronization further comprises: waiting for the write reorder queue empty status indicator from the memory buffer chips; decrementing the first time period while waiting for the first time period to expire based on determining that the replay is not in progress; based on determining that synchronization is not reestablished for the second time period, determining whether the memory buffer chip out-of-sync condition exists; and based on determining that the memory buffer chip out-of-sync condition does not exist, repeating the second stage of reestablishing synchronization for a number of times before advancing to the third stage of reestablishing synchronization or declaring a failure.
 3. The system of claim 1, wherein performing the first stage of reestablishing synchronization further comprises: decrementing the first time period while waiting for the first time period to expire based on determining that the replay is not in progress.
 4. The system of claim 1, wherein selectively stopping new traffic on the plurality of channels further comprises: stopping the new traffic on the plurality of channels unless a skip counter is enabled and not exceeded.
 5. A computer implemented method for reestablishing synchronization across multiple channels in a memory system within a computer, the method comprising: receiving an out-of-synchronization indication associated with at least one of a plurality of channels in the memory system within the computer, wherein the plurality of channels within the computer each provide communication with a memory buffer chip and a plurality of memory devices; performing, by a memory control unit in communication with the channels within the computer, a first stage of a three stage process of reestablishing synchronization comprising: selectively stopping new traffic on the plurality of channels; waiting for a first time period to expire; resuming traffic on the plurality of channels based on the first time period expiring; verifying that synchronization is reestablished for a second time period; and based on determining that synchronization is not reestablished for the second time period, repeating the first stage of reestablishing synchronization for a number of times before performing a second stage of the three stage process of reestablishing synchronization; performing, by the memory control unit, the second stage of reestablishing synchronization comprising: stopping new traffic on the plurality of channels; waiting for outstanding traffic on the plurality of channels to complete; resuming traffic on the plurality of channels based on the first time period expiring; and verifying that synchronization is reestablished for the second time period; and performing a third stage of the three stage process of reestablishing synchronization based on determining that a memory buffer chip out-of-sync condition exists, the third stage comprising: stopping new traffic on the plurality of channels; waiting for outstanding traffic on the plurality of channels to complete; waiting for a write reorder queue empty status indicator from the memory buffer chips before sending a synchronization command; sending the synchronization command to the memory buffer chips on each of the channels; waiting for a third time period to expire; verifying that a replay did not occur during the third time period, wherein the replay comprises a recovery retransmission sequence from a replay buffer that causes a faulty channel to go out of synchronization with non-faulty instances of the channels; waiting a fourth time period before resuming traffic on the plurality of channels; resuming traffic on the plurality of channels; verifying that synchronization is reestablished for the second time period; and based on determining that synchronization is not reestablished for the second time period, repeating the third stage of reestablishing synchronization for a number of times before declaring a failure.
 6. The method of claim 5, wherein performing the second stage of reestablishing synchronization further comprises: waiting for the write reorder queue empty status indicator from the memory buffer chips; decrementing the first time period while waiting for the first time period to expire based on determining that the replay is not in progress; based on determining that synchronization is not reestablished for the second time period, determining whether the memory buffer chip out-of-sync condition exists; and based on determining that the memory buffer chip out-of-sync condition does not exist, repeating the second stage of reestablishing synchronization for a number of times before advancing to the third stage of reestablishing synchronization or declaring a failure.
 7. The method of claim 5, wherein performing the first stage of reestablishing synchronization further comprises: decrementing the first time period while waiting for the first time period to expire based on determining that the replay is not in progress.
 8. A computer program product for reestablishing synchronization across multiple channels in a memory system within a computer, the computer program product comprising: a non-transitory machine readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: receiving an out-of-synchronization indication associated with at least one of a plurality of channels in the memory system within the computer, wherein the plurality of channels within the computer each provide communication with a memory buffer chip and a plurality of memory devices: performing, by a memory control unit in communication with the channels within the computer, a first stage of a three stage process of reestablishing synchronization comprising: selectively stopping new traffic on the plurality of channels; waiting for a first time period to expire; resuming traffic on the plurality of channels based on the first time period expiring; verifying that synchronization is reestablished for a second time period; and based on determining that synchronization is not reestablished for the second time period, repeating the first stage of reestablishing synchronization for a number of times before performing a second stage of the three stage process of reestablishing synchronization; performing, by the memory control unit, the second stage of reestablishing synchronization comprising: stopping new traffic on the plurality of channels; waiting for outstanding traffic on the plurality of channels to complete; resuming traffic on the plurality of channels based on the first time period expiring; and verifying that synchronization is reestablished for the second time period; and performing a third stage of the three stage process of reestablishing synchronization based on determining that a memory buffer chip out-of-sync condition exists, the third stage comprising: stopping new traffic on the plurality of channels; waiting for outstanding traffic on the plurality of channels to complete; waiting for a write reorder queue empty status indicator from the memory buffer chips before sending a synchronization command; sending the synchronization command to the memory buffer chips on each of the channels; waiting for a third time period to expire; verifying that a replay did not occur during the third time period, wherein the replay comprises a recovery retransmission sequence from a replay buffer that causes a faulty channel to go out of synchronization with non-faulty instances of the channels; waiting a fourth time period before resuming traffic on the plurality of channels; resuming traffic on the plurality of channels; verifying that synchronization is reestablished for the second time period; and based on determining that synchronization is not reestablished for the second time period, repeating the third stage of reestablishing synchronization for a number of times before declaring a failure.
 9. The computer program product of claim 8, wherein performing the second stage of reestablishing synchronization further comprises: waiting for the write reorder queue empty status indicator from the memory buffer chips; decrementing the first time period while waiting for the first time period to expire based on determining that the replay is not in progress; based on determining that synchronization is not reestablished for the second time period, determining whether the memory buffer chip out-of-sync condition exists; and based on determining that the memory buffer chip out-of-sync condition does not exist, repeating the second stage of reestablishing synchronization for a number of times before advancing to the third stage of reestablishing synchronization or declaring a failure.
 10. The computer program product of claim 8, wherein performing the first stage of reestablishing synchronization further comprises: decrementing the first time period while waiting for the first time period to expire based on determining that the replay is not in progress. 